The general idea of FDF is similar to HTML forms. The difference is
basically the format how data is transmitted to the server when the submit
button is pressed (this is actually the Form Data Format) and the format
of the form itself (which is the Portable Document Format, PDF).
Processing the FDF data is one of the features provided by the fdf
functions. But there is more. One may as well take an existing PDF form
and populated the input fields with data without modifying the form
itself. In such a case one would create a FDF document
(fdf_create()) set the values of each input field
(fdf_set_value()) and associate it with a PDF form
(fdf_set_file()). Finally it has to be sent to the
browser with MimeType application/vnd.fdf. The Acrobat
reader plugin of your browser recognizes the MimeType, reads the
associated PDF form and fills in the data from the FDF document.

If you look at an FDF-document with a text editor you will find a
catalogue object with the name FDF. Such an object may
contain a number of entries like Fields,
F, Status etc..
The most commonly used entries are Fields which points
to a list of input fields, and F which contains the
filename of the PDF-document this data belongs to. Those entries are
referred to in the FDF documentation as /F-Key or /Status-Key.
Modifying this entries
is done by functions like fdf_set_file() and
fdf_set_status(). Fields are modified with
fdf_set_value(), fdf_set_opt() etc..